Questions & Answers
(adapted from NSSL)
- What causes lightning?
Lightning originates around 15,000 to 25,000 feet above sea level
when raindrops are carried upward until some of them convert to ice.
For reasons that are not widely agreed upon, a cloud-to-ground
lightning flash originates in this mixed water and ice region. The
charge then moves downward in 50-yard sections called step leaders.
It keeps moving toward the ground in these steps and produces a
channel along which charge is deposited. Eventually, it encounters
something on the ground that is a good connection. The circuit is
complete at that time, and the charge is lowered from cloud to
The flow of charge (current) produces a luminosity that is very
much brighter than the part that came down. This entire event
usually takes less than half a second.
- Where does lightning usually strike?
Lightning comes from a parent cumulonimbus cloud. These thunderstorm
clouds are formed wherever there is enough upward motion,
instability in the vertical, and moisture to produce a deep cloud
that reaches up to levels somewhat colder than freezing.
These conditions are most often met in summer. In general, the US
mainland has a decreasing amount of lightning toward the northwest.
Over the entire year, the highest frequency of cloud-to-ground
lightning is in Florida between Tampa and Orlando. This is due to
the presence, on many days during the year, of a large moisture
content in the atmosphere at low levels (below 5,000 feet), as well
as high surface temperatures that produce strong sea breezes along
the Florida coasts. The western mountains of the US also produce
strong upward motions and contribute to frequent cloud-to-ground
lightning. There are also high frequencies along the Gulf of Mexico
coast westward to Texas, the Atlantic coast in the southeast US, and
inland from the Gulf. Regions along the Pacific west coast have the
least cloud-to-ground lightning.
Flashes that do not strike the surface are called cloud flashes.
They may be inside a cloud, travel from one part of a cloud to
another, or from cloud to air.
- Can lightning be detected?
Since the 1980s, cloud-to-ground lightning flashes have been
detected and mapped in real time across the entire US by several
In 1994, the networks were combined into one national network
consisting of antennas that detect the angle from ground strike
points to an antenna (direction-finder antenna), that detect the
time it took for them to arrive at an antenna (time-of-arrival
method), or a combination of both detection methods. The network is
operated by Global Atmospherics,
Flashes have also been detected from space during the past few
years by an optical
sensor. This experimental satellite covers the earth twice a day
in tropical regions. The satellite also detects flashes that do not
strike the ground, but cannot tell the difference between ground
strikes and cloud flashes. Click
here to visit the Boundary Waters Lightning Detection System.
- How many flashes are there?
Over the continental 48 states, an average of 20,000,000
cloud-to-ground flashes have been detected every year since the
lightning detection network covered all of the continental US in
1989. In addition, about half of all flashes have more than one
ground strike point, so at least 30 million points on the ground are
struck on the average each year in the US. Besides cloud-to-ground
flashes, there are roughly 5 to 10 times as many cloud flashes as
there are to ground.
- What types of damage can lightning cause?
Cloud-to-ground lightning can kill or injure people by direct or
indirect means. The lightning current can branch off to a person
from a tree, fence, pole, or other tall object. It is not known if
all people are killed who are directly struck by the flash itself.
In addition, flashes may conduct their current through the ground to
a person after the flash strikes a nearby tree, antenna, or other
tall object. The current also may travel through power or telephone
lines, or plumbing pipes to a person who is in contact with an
electric appliance, telephone, or plumbing fixture.
Similarly, objects can be directly struck and this impact may
result in an explosion, burn, or total destruction. Or, the damage
may be indirect when the current passes through or near it.
Sometimes, current may enter a building and transfer through wires
or plumbing and damage everything in its path. Similarly, in urban
areas, it may strike a pole or tree and the current then travels to
several nearby houses and other structures and enter them through
wiring or plumbing.
- How to stay safe when lightning is around: use the 30-30 Rule!
The best defense is to plan ahead and avoid exposure to lightning
when a thunderstorm occurs. Know where safe shelter is located and
leave enough time to reach safe shelter before your danger level is
high. Don't be an isolated tall object, and don't be connected to
anything that may be an isolated tall object.
NSSL's scientists and collaborators did a study to find out how
close is too close. They studied thunderstorms in Florida and
Oklahoma and found that 80% of lightning strokes would be within 2
miles of each other in Florida and within 6 miles of each other in
Oklahoma. One way to find the distance to lightning is to use the
'flash-to-bang' method. Count 5 seconds for every mile the
lightning is away from you. Safe shelter must be reached
completely in all situations before a flash is within 6 miles, which
is 30 seconds flash-to-bang. Most of the time if you can hear
thunder you are no longer safe. Lightning safety is also considered
But there is often blue sky in some direction while lightning is
occurring nearby, and it may not be raining, so pay much more
attention to the lightning than the rain. A particularly difficult
situation is the first flash from a storm--watch for a storm that is
growing quickly, such as when a storm is becoming very dark at its
base or is growing very tall. An equally dangerous situation is when
a storm appears to be finished, and only light rain and/or
occasional thunder are heard, but the cloud overhead continues to be
fairly dark. The most common situation for a lightning death or
injury in Florida was found NOT to be in the heaviest rain area with
lots of flashes, but after or before the time when rain and
lightning was the most intense. So, the weak storm without too many
flashes, at the edge of a larger storm, or early or late in the life
of a storm is most dangerous.
The best shelter is a substantial building that has plumbing and
wiring--in other words, one that is used or lived in by people for a
major portion of the day. A very unsafe building for lightning has
only a roof and some supports, but no wiring or pipes extending into
the ground. A vehicle with a metal roof provides good shelter, and
is much better than being in the open or in an ungrounded building,
but is not as good as being in a building that is grounded by wires